Two inherently contradictory objectives are being pursued when generating energy through combustion of fossil fuels. Firstly, the efficiency which can be achieved is supposed to be lowered in order to save on fuel energy and to reduce the emissions of CO2. Secondly, the lowest possible emissions of pollutants, i.e. in particular NOx and CO, need to be realized.
One possible way of improving the efficiency of a combustion process is based on strong preheating of the combustion air. As a result, the combustion takes place at higher flame temperatures, and the energy of the hot combustion gases is ultimately transferred back to the combustion air in recuperative or regenerative heat exchangers. However, one drawback of the high air preheating temperatures is higher peak temperatures in the flame, which have a disastrous effect on the thermal formation of nitrogen oxides.
Winning and Wunning et al, in “Flammenlose Oxidation von Brennstoff mit hochvorgewärmter Luft” [Flameless oxidation of fuel with highly preheated air], Chemical Ingenieur Tech. 63(12), 1243-1245 (1991), and in “Flameless Oxidation to Reduce Thermal NO-Formation”, Prog. Energy Combust. Sci. 23 (1997), have proposed a novel concept for combustion under atmospheric conditions, i.e. in particular at a pressure which is not elevated or is only slightly elevated. In this case, a combustible mixture of fuel and oxidizing agent, in particular air, is brought to temperatures above the spontaneous ignition threshold value. This combustion technique has become known as combustion without the appearance of a flame, mild combustion, colorless combustion or high-temperature combustion. The underlying concept of this new technique consists in the generation of a highly dilute reaction mixture of fuel and oxidizing agent and flue gas, which is held at a temperature above the spontaneous ignition threshold value. Flue gas recirculation on the one hand dilutes the reaction mixture and on the other hand delivers the energy required for preheating to a temperature above the spontaneous ignition threshold value. In this combustion process, the flame front which has hitherto typically occurred in burners is replaced by an invisible volumetric flame which is distributed approximately uniformly through the combustion volume.
One problem with realizing a method of this type is the recirculation of a sufficient quantity of flue gas. Typically, it is necessary to recirculate at least the same volumetric flow of flue gas as the volumetric flow of fresh combustible fluid mixture which is supplied. In particular in the case of gas turbine applications, this involves temperatures of, for example, 1400° C. at the turbine entry. The recirculation is also difficult to implement because burner systems usually have a not inconsiderable pressure loss, so that the total pressure of the flue gas to be recirculated is below that of the fresh gas supplied. Overall, the recirculation of sufficient quantities of flue gas using conventional recirculation techniques is only possible with considerable difficulty, if at all.